A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs) or other devices. In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the device. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate.
In general, a single substrate will contain a network of features that may be adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing a pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or transverse to this direction.
It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Manufacturing devices, such as semiconductor devices, typically involves processing a substrate (e.g., a semiconductor wafer) using a number of fabrication processes to form various features and often multiple layers of the devices. Such layers and/or features are typically manufactured and processed using, e.g., deposition, lithography, etch, chemical-mechanical polishing, and ion implantation. Multiple devices may be fabricated on a plurality of dies on a substrate and then separated into individual devices. This device manufacturing process may be considered a patterning process.
A patterning process involves a pattern transfer step, such as optical and/or nano-imprint lithography using a lithographic apparatus, to provide a pattern on a substrate and typically, but optionally, involves one or more related pattern processing steps, such as resist development by a development apparatus, baking of the substrate using a bake tool, etching the pattern by an etch apparatus, etc. Further, one or more metrology processes are involved in the patterning process.
Metrology processes are used at various steps during a patterning process to monitor and/or control the process. For example, metrology processes are used to measure one or more characteristics (or parameters) of a substrate, such as a relative location (e.g., registration, overlay, alignment, etc.) or dimension (e.g., line width, critical dimension (CD), thickness, etc.) of features formed on the substrate during the patterning process, such that, for example, the performance of the patterning process can be determined from the one or more characteristics. If the one or more characteristics are unacceptable (e.g., out of a predetermined range for the characteristic(s)), one or more variables of the patterning process may be redesigned or altered, e.g., based on the measurements of the one or more characteristics, such that substrates manufactured by the patterning process have acceptable characteristics.
With the advancement of lithography and other patterning process technologies, the dimensions of functional elements of devices have been reduced while the amount of the functional elements per device has increased. The requirement of accuracy in terms of overlay, CD etc. has become more and more stringent. Error will inevitably be produced in the patterning process. The error may cause a problem in terms of the functioning of the device, including failure of the device to function or one or more electrical problems of the functioning device. Accordingly, it is desirable to be able to characterize one or more of these errors and take steps to design, modify, control, etc. a patterning process to reduce or minimize one or more of these errors.
For example, accurate and robust overlay measurements are a significant factor in improving the yield of devices for a given substrate. In known methods for overlay measurement, targets in the form of, e.g., periodic gratings are placed at different positions on a substrate, such as a semiconductor wafer. The targets are illuminated by radiation, which radiation is scattered by the target and received by a detector and a pupil is produced. Analysis of the intensity of pixels of the pupil provides an estimation of the overlay imparted when the targets were fabricated. This estimation of overlay may be transposed across the remainder of the substrate.
Several approaches for overlay metrology are known and some are briefly described below:                In diffraction-based overlay (DBO) the overlay is extracted from the asymmetry of the ±1st orders of radiation scattered off of a metrology target. The illumination spot formed by the radiation beam under-fills the target and the measurement is performed in the Fourier (or pupil) plane.        In diffraction-based overlay on small targets (μDBO) the overlay is again extracted from the asymmetry of the ±1st orders of a radiation scattered off of a metrology target. The illumination spot formed by the radiation beam over-fills the target and the measurement is performed in the image plane. The image plane is chosen (instead of the Fourier plane) in order to facilitate the removal of target edge effects.        